Functional fluid compositions

ABSTRACT

Phosphate ester base functional fluid compositions containing small amounts of water and a mixture of high and low molecular weight polymers of the alkyl esters of Alpha - Beta unsaturated aliphatic mono-carboxylic acids as viscosity index improvers inhibit erosion of material and damage to mechanical members in a hydraulic system.

United States Patent 1 Richard, Jr.

[ 51 Feb. 27, 1973 [7 51 Inventor:

[ FUNCTIONAL FLUID COMPOSITIONS William R. Richard, Jr., Kirkwood, Mo.

[52] U.S. Cl ..252/78, 252/49.8

[51] Int. Cl. ..C09k 3/00 [58] Field of Search ..252/7378, 49.9, 252/389 [56] References Cited UNITED STATES PATENTS 2,470,792 5/1949 Schlesinger et al ..252/78 2,933,449 4/1960 Moreton OTHER PUBLICATIONS Acrylic Ester Polymers, Luskin & Myers reprint from Encyclopedia of Polymer Science & Technology, Vol. I, pgs. 317-318. Cavitational Erosion & Means for its Prevention," Bogachez & Mints U.S. Clearing House, pgs. 99103,107, & 112. The Electrochemical Approach to Cavitation Damage & Its Prevention Preiser et al. Corrosion, Vol. 17 (1961) pgs. 535T, 536T, 540T, & 541T.

Primary Examiner--Leon D. Rosdol Assistant Examinerl-iarris A. Pitlick Attorney-Neal E. Willis, J. E. Maurer and William T. Black 7 [57] ABSTRACT Phosphate ester base functional fluid compositions containing small amounts of water and a mixture of high and low molecular weight polymers of the alkyl esters of a-B unsaturated aliphatic mono-carboxylic acids as viscosity index improvers inhibit erosion of material and damage to mechanical members in a hydraulic system.

15 Claims, No Drawings FUNCTIONAL FLUID COMPOSITIONS This invention relates to functional fluid compositions used in hydraulic power systems as lubricants and/r heating/cooling systems having an ability to inhibit and control damage to mechanical members of the system in contact with said fluid and more particularly to functional fluid compositions comprising certain phosphate ester containing a mixture of two different molecular weight viscosity index improvers which are polymers of the alkyl esters of a-fl unsaturated aliphatic monocarboxylic acids and from 0.1 to 2.0 percent by weight water.

Many different types of materials are utilized as functional fluids and functional fluids are used in many different types of applications. Thus, such fluids have been used as electronic coolants, atomic reactor coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids and as filter mediums for air conditioning systems. In many of these. uses'there have been reports of damage to the fluid during use and to mechanical members, especially metallic members, in contact with the fluid as evidenced by a loss of weight of such members. Thus, damage has been reported in aircraft hydraulic systems, gas turbine bearings, jet turbine control systems, steam turbine bearings and steam turbine control systems.

One particularly undesirable condition which exists during the use of a functional fluid and which can cause damage is erosion of material. One mechanism causing damage could be cavitation which can be described as a phenomenon resulting from movement of a fluid at a given pressure to a lower pressure, the lower pressure being obtained by an increase in fluid velocity at a given point or zone in the pressure system. Thereduction in pressure is of such a magnitude as to cause cavitation severe enough to damage mechanical members and/or the fluid.

While there are many undesirable results caused by damage, one important aspect of the problem of damage is the effect on hydraulic systems and fluids experiencing such damage. For example, the structural mechanical parts in a hydraulic system, such as pumps and valves, exhibit a marked decrease in strength, and the geometry of the parts is altered. These changes in the case of pumps cause a decrease in the efficiency of operation and in the case of valves can cause faulty operation, excessive leakage or even hazardous conditions. This damage necessitates premature overhaul of mechanical parts which is both costly and time consuming. Furthermore, as damage occurs the metal from metallic mechanical parts in contact with the functional fluid contaminates the functional fluids requiring premature draining of the fluids from the system, filter clogging and replacement, and can cause a change in It is known that the addition of water to phosphate ester based hydraulic fluids will effectively inhibit and control damage in a hydraulic system when the water is present in amounts of from 0.3 to 10 volume percent of the total volume of the fluid. Although the addition of water in such amounts to functional fluids will inhibit and control damage to valves and other mechanical members of a hydraulic system, various undesirable side effects are noted. Thus, for example, large amounts of water present in the system can lead to the hydrolysis of the phosphate esters giving rise to amounts of the free acidic groups in the fluid which causes corrosion and other undesirable effects. The increase in the total acidity of the phosphate ester based function thus necessitates frequent draining of the hydraulic fluids after relatively short periods of operation.

It is therefore an object of this invention to provide phosphate ester based hydraulic fluids which are capable of inhibiting or preventing the damage to mechanical member by cavitation or erosion and which can contain small amounts of water in order to prevent the undesirable side effects such as hydrolysis of the ester and increases in the acid number of such fluids.

Further objects will be apparent from the following description of the invention.

It has now been found that damage herein defined to include damage toa functional fluid and to mechanical members in contact with said fluid, can be effectively reduced or inhibited in the phosphate ester based functional fluid systems by the incorporation of small amounts of water into a functional fluid, together with a mixture of a high molecular weight and a low molecular weight viscosity index improver. The incorporation of these mixtures into phosphate ester based functional fluids improves the ability of such fluids to inhibit damage without adversely affecting the other properties of such fluids such as viscosity, oxidative and thermal stability, corrosion resistance in the presence of metal parts and the lubricating qualities of the functional fluid.

It has unexpectedly been discovered that the amount of water needed to decrease the erosion of material or metal erosion effects of phosphate ester based hydraulic fluids can be greatly diminished if a mixture of high and low molecular weight viscosity index improvers are employed in such fluid formulations. It has further been unexpectedly found for a given amount of water in a phosphate ester based hydraulic fluid that by employing a mixture of the high and low molecular weight viscosity index improvers that such mixtures enhance the ability of the water to reduce or inhibit erosion of mechanical members of the system.

More specifically it has been found that functional fluid compositions containing (I) phosphate esters and mixtures thereof wherein the phosphate esters are represented by the formula wherein each is individually an alkyl group containing from 2 to 12 carbon atoms, an alkoxy alkyl group, phenyl groups and alkyl-substituted phenyl groups contain up to 16 carbon atoms wherein the alkyl groups contain from one to carbon atoms with the proviso that the ratio of alkyl groups represented by R to phenyl and alkyl-substituted phenyl groups is at least 2 to 1, (ll) small amounts of water and (III) as a viscosity index improver a mixture of (A) a higher molecular weight viscosity index improver with (B) a lower molecular weight viscosity index improver inhibits and controls erosion of material and metal erosion to metal members in contact with such compositions.

The total amount of viscosity index improver employed in the compositions of the instant invention can range from about 4 to about 20 parts per 100 parts of the total composition. The ratio of high molecular weight viscosity index improver (A) to the lower molecular weight viscosity index improver (B) can be from 1 to 20 to 20 to 1. It is particularly preferred that the ratio of(A) to (B) be from about 2.5/7.5 to 7.5/2.5. It is even more preferred that the ratio of (A) to (B) be from 6 to 4 to 4 to 6.

It has been found that where a mixture of the viscosity index improvers are employed, a great improvement of the cavitation erosion or damage resistance can be obtained with as little as 0.1 part of water per 100 parts of the total composition. The amount of water employed in the present composition can range as high as 2.0 parts per 100 parts of the total composition. One obtains an improvement over prior art compositions containing 0.3 to 2.0 parts water per 100 total parts of the composition by employing the mixture of viscosity index improvers as set forth above.

As has been set forth above it is critical to the compositions of this invention to employ as the viscosity index improver a mixture of (A) a high molecular weight polymer of an alkyl ester of an alpha-beta unsaturated aliphatic monocarboxylic acid with (B) a low molecular weight polymer of an alpha-beta unsaturated aliphatic monocarboxylic acid. Thus in the compositions of this invention the mixture can comprise a mixture of high and low molecular weight polymers of alkyl esters of the same or different (alpha-beta) unsaturated aliphatic monocarboxylic acid. In these mixtures the alkyl group of the ester can be the same or different. It is particularly preferred to employ a mixture of high and low molecular weight polymeric viscosity index improvers wherein the alkyl group of the ester be different, for example, a mixture of poly(butylmethacrylate) and poly(n-hexylmethacrylate), or a mixture of poly(nonylacrylate) and poly(ethylacrylate). By the term high molecular weight polymer is meant a polymer having an average molecular weight of from 15,000 to about 40,000. By the term low molecular weight polymer is meant a polymer having an average molecular weight of from about 2,500 to about 12,000. It is by the employment of such mixtures that the startling and unexpected results are obtained with the functional fluid compositions of this invention.

The polymeric viscosity index improvers employed in the compositions of this invention are the polymers of alkyl esters of (alpha-beta) unsaturated monocarboxylic acids having the formula 'wherein R1 and R are eaClTiFdiViHHaTIyhydrOgen or an alkyl group containing from 1 to 10 carbon atoms and R is a C, to C alkyl group. Illustration of the alkyl groups represented by R, R and R" within their definitions as given above are for example methyl, ethyl, propyl, butyl, t-butyl. isopropyl. 2-ethylhexyl, hexyl, decyl, undecyl, dodecyl, octadecyl, tetracosyl, and the like. These polymers include, for example, poly(butylmethacrylates), poly(hexylmethacrylates), poly(octylacrylates), poly(dodecylacrylates) and polymers wherein the ester is a mixture of compounds obtained by esterifying the m5 unsaturated monocarboxylic acid with a mixture of monoalcohols containing from I to 32 carbon atoms.

Particularly suitable polymeric viscosity index improvers suitable for use in the compositions of this invention are the polyalkylmethacrylates and acrylates, and in general are those resulting from the polymerization of alkylmethacrylates or alkylacrylates in which the alkyl groups have from 1 to 24 carbon atoms and preferably from four to 24 carbon atoms. The alkyl groups may be mixtures such as derived from a mixture of alcohols in which case there may be included some alkyl groups having as low as one carbon atom and as high as about 32 carbon atoms, e.g., dotriacontyl. The number of carbon atoms in the alkyl group should preferably be such that the polymer is compatible with the particular fluid used. Usually, it will be satisfactory for the alkyl group of the methacrylate polymer to be from about four to 10 carbon atoms. The alkyl group may be a branched chain or isoalkyl, but it is preferably normal alkyl. The molecular weight of the polymerized alkylmethacrylate is critical if inhibition of metal damage or erosion of material is to be obtained.

The preferred esters of phosphoric acid which can be employed as base stocks in the compositions of the instant invention are those wherein the groups represented by R are alkyl, alkoxyalkyl, phenyl, or alkyl-substituted phenyl. The preferred base stocks are hereinafter referred to generically as phosphates and include trialkyl phosphates, tripheny and/or alkyl-substituted phenyl phosphates and mixed phenyl and/or alkyl-substituted phenyl phosphates. The alkyl groups preferred are those containing from two to 12 carbon atoms with the total number of carbon atoms in the trialkyl phosphates being from 12 to 36 carbon atoms. These alkyl groups are for example, ethyl, propyl, isopropyl, butyl, hexyl, 2-ethylhexyl, dodecyl, decyl, and the like. The alkyl-substituted phenyl groups represented by the R's in Formula I are those containing up to 16 carbon atoms and the alkyl groups contain 1 to 10 carbon atoms provided that the total number of carbon atoms in all of the alkyl groups attached to any one phenyl group be at most 10 carbon atoms. Illustrative of these alkyl-substituted phenyl groups are for example, methylphenyl, e'thylphenyl, dimethyl, propylphenyl, nonylphenyl, decylphenyl, dipentylphenyl, butylhexylphenyl, and the like. The alkoxyalkyl groups are for example, those having the formula H(C,,H ,,O), C ll wherein n is an integer having a value of from 1 to 10, a is an integer having a value of from 2 to 10, and x is an integer of l to 10, preferably x is 1. These alkoxyalkyl groups are for example methoxyethyl, ethoxyethyl, propoxyethyl, ethoxypropyl, ethoxypentyl, propoxydecyl, nonyloxyethyl, octyloxybutyl and the like. Typical examples of these phosphate esters are for example, dibutylphenyl phosphate, triphenyl phosphate, tricresyl phosphate, 'tributyl phosphate, tri-2-ethylhexyl phosphate, trioctyl phosphate, and mixtures of the above phosphates such as mixtures of tributyl phosphate and tricresyl phosphate, mixtures of isooctyl diphenyl phosphate and Z-ethylhexyl diphenyl phosphate, and mixtures of trialkyl phosphates and tricresyl phosphates and the like. The particularly preferred phosphate esters are those which remain liquid at temperatures of about 30C.

The compositions of the present invention can be prepared by methods known to those skilled in the art. For example by blending in mixing equipment, such as with lightning stirrers in stainless steelequipment, the phosphate ester and the viscosity index improver can be blended and the quantity of water added to this mixture with stirring until a uniform mixture is obtained.

The compositions of the instant invention are useful as hydraulic fluids for aircrafts, etc.

The following examples serve to better illustrate the invention. All parts are parts by weight unless otherwise expressly set forth.

In the examples as given in Table l, the damage tests were run using mild steel specimens in about 750-l,000 cc of fluid at a fluid temperature of about 137F. The metal specimen was vibrated in the fluid at kc with a yield 0.002-inch amplitude for a period of 45 minutes. In the examples theviscosity index improver A was a butylmethacrylate polymer having an average molecular weight of 29,000 and viscosity indeximprover B was a hexylmethacrylate polymer having a molecular weight of 5600. In the examples all of the solutions tested are compared insofar as erosion damage to mechanical members is concerned with a fluigiwhich contains no viscosity index improver, such standard tests being arbitrarily given the value of 100. In Table I, under the heading Base Stock the abbreviations have the following meaning:

DBPP dibutylphenyl phosphate TBP tri-n-butyl phosphate CDP cresyldiphenyl phosphate and TCP tricresyl phosphate TABLE I V.I. improver parts Water Relative Base stock Parts A B parts damage Example 1...... DBPP 92 4.0 4.0 .12 59.1

Con rolA.- DBPP 92 8.0 .12 69. ControlB.. DBPP 92 8.0 .12 92. Exun1p1e2 DBPP 92 4.0 4.0 .18 46. ControlA. DBPP 92 8.0 .18 63. Control 13.. DBPP 92 8.0 .18 99. Exampleii TBP/CDP 71.0/24.0 2.5 2.5 .25 64. ControlA.. TBP/CDP 71.0/24.0 5.0 .25 82. Control B.. TBP/CDP 71.0/24.0 5.0 .27 71. Exsmple4 TBP/CDP 70.4/21.6 4.0 4.0 .12 41. ControlA.- TBP/CDP 70.4/21.6 8.0 .12 62. ControlB.. TBP/CDP 70.4/21.6 8.0 .12 95. Example5 TBP/CDP 70.4/21.6 4.0 4.0 .18 55. Control A. TBP/CDP 70.4/21.6 8.0 .19 64. Control B TBP/CDP 70.4/21.6 8.0 .19 70. Examples TBP/CDP 70.4/21.6 4.0 4.0 .31 48. ControlA.- TBP/CDP 70. .6 8.0 .29 64.

Con roi 13.. TBP/CDP 70.4/21.6 so .20 60.7 Examnle7 TBP/CDP 10. 4/21.s a. 25 3.25 .20 84.3 Contr0lA.- TBP/CDP 70.4/21.6 6.5 .20 114.7 ControlB.. TBP/CDP 70.4/21.s 6.5 .20 96.6 ExamploB DBPP 92 4.0 4.0 .5 33.4 Control A.. DBPP 02 8.0 .5 520 Control IL. D131? 92 8.0 .5 57.0 Example 0...... TBP 92 4.0 4. 0 .51 33.0 ControlA TBP n2 8.0 .51 58.4 Control IL. TBP 92 8 0 .51 45.5

The test method as employed to determine relative damage has been found to correlate quite well to actual test runs on simulated hydraulic system test stands, such as the F airey Test Stand. In addition, the hydraulic system test stands for determining damage have correlated quite well with the hydraulic system of commercial aircraft where damage levels have been determined. Functional fluid compositions of this invention with a viscosity index improver system comprising the mixture of high molecular weight and low molecular weight polymers with water concentrations sufficient to inhibit the control damage have been evaluated in actual hydraulic systems in test stands and commercial aircraft and have been found to effectively inhibit damage and are far superior to the neat fluids with additive amounts of water but employing only a single polymer as the viscosity index improver.

The data in the examples demonstrate the significant inhibition of damage obtained by the incorporation of the mixture of viscosity index improvers into a base stock. In addition, the physical properties and the perfonnance characteristics such as lubricity, fire resistance, and viscosity were essentially unaffected by the additive, an important consideration since a base stock is selected for a given fluid system because of its physical properties or characteristics and deviations from these properties and characteristics can bring about inferior fluid performance.

As a result of the excellent inhibition and control of damage utilizing the functional fluid compositions within the scope of this invention, improved hydraulic pressure devices can be prepared in accordance with this invention which comprise in combination a fluid chamber and an actuating fluid composition in said chamber, said fluid comprising base stock compositions hereinbefore described. In such a system, the parts which are so lubricated include the frictional sur-. faces of the source of power, namely the pump, valves, operating pistons and cylinders, fluid motors, and in some cases, for machine tools, the ways, tables and slides. The hydraulic systernmay be of either the constant volume or the variable volume type of system.

The pumps may be of various types, including centrifugal pumps, jet pumps, turbine vane, liquid piston gas compressors, piston-type pump, more particularly the variable-stroke piston pump, the variable-discharge or variable displacement piston pump, radial-piston pump, axial-piston pump, in which a pivoted cylinder block is adjusted at various angles with the piston assembly, for example, the Vickers Axial-Piston Pump, or in which the mechanism which drives the pistons is set at an angle adjustable with the cylinder block; geartype pump, which may be spur, helical or herringbone gears, variations of internal gears, or a screw pump; or vane pumps. The valves may be stop valves, reversing valves, pilot valves, throttling valves, sequence valves, relief valves, servo valves, non-return valves, poppet valves or unloading valves. Fluid motors are usually constantor variable-discharge piston pumps caused to rotate by the pressure of the hydraulic fluid of the system with the power supplied by the pump power source. Such a hydraulic motor may be used in connection with a variable-discharge pump to form a variablespeed transmission. It is, therefore, especially important that the frictional parts of the fluid system which are lubricated by the functional fluid be protected from damage. Thus, damage brings about seizure of frictional parts, excessive wear and premature replacement of parts.

The fluid compositions of this invention when utilized as a functional fluid can also contain dyes, pour point depressants, metal deactivators, acid scavengers, such as for example, an organic compound which contains at least one epoxide group such as phenylglycidyl ether, pinene oxide, styrene oxide, glycidyl cyclohexyl ether, glycidyl epoxycyclohexyl ether, diglycidyl ether, glycidyl isopropyl ether, butadiene dioxide cyclohexylene oxide, bis-epoxycyclohexyl adipate, 3,4-epoxycyclohexylcarboylate of 3,4-epoxycyclohexane and the like, antioxidants, defoamers in concentration sufficient to impart antifoam properties, such as from about 10 to about 50,000 percent) parts per million, lubricity agents and the like.

While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims.

What is claimed is:

l. A hydraulic fluid composition comprising by weight I. from about 80 to 96 percent of phosphate esters and mixtures thereof having the formula wherein each R is individually an alkyl or alkoxy alkyl group containing from two to 12 carbon atoms, a phenyl group or an alkyl-substituted phenyl group containing up to 16 carbon atoms wherein the alkyl substituents contain from one to carbon atoms, provided that the ratio of alkyl groups represented by R to phenyl and alkyl-substituted phenyl groups is at least 2 to 1, II. from about 4 to about 20 percent of a viscosity index improver consisting essentially of polymers of alkyl esters of 01-13 unsatura ted aliphatic monocarboxyllc acids, said vrscoslty index 1mprover being a mixture of a. a high molecular weight polymer wherein the molecular weight is from 15,000 to 40,000 and b. a low molecular weight polymer having a molecular weight of from 2,500 to 12,000; provided that the ratio of (a) to (b) is from 1 to 20 to 20 to l and Ill. from 0.1 to 2.0 percent by weight water.

2. A composition as claimed in claim 1 wherein the viscosity index improver is a mixture of polyalkylmethacrylates, and the alkyl ester groups of (a) and (b) are different.

3. A composition as claimed in claim 1 wherein the viscosity index improver is a mixture of polyalkylacrylates, and the alkyl ester groups of (a) and (b) are different.

4. A composition as claimed in claim 2 wherein the phosphate ester is dibutylphenyl phosphate.

5. A composition as claimed in claim 2 wherein the phosphate ester is tributyl phosphate.

6. A composition as claimed in claim 2 wherein the phosphate ester is a mixture of tributyl phosphate and tricresyl phosphate.

7. A composition as claimed in claim 2 wherein the phosphate ester is a mixture of a trialkyl phosphate and cresyldiphenyl phosphate.

8. A composition as claimed in claim 1 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly( hexylmethacrylate 9. A composition as claimed in claim 2 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).

10. A composition as claimed in claim 4 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).

11. A composition as claimed in claim 5 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).

12. A composition as claimed in claim 6 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly( hexylmethacrylate).

13. A composition as claimed in claim 7 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).

14. In the method of controlling cavitation damage in hydraulic systems while operating a hydraulic device wherein a displacing force is transmitted to a displaceable member by means of a hydraulic fluid, the improvement which comprises employing as said fluid a composition of claim 1.

15. In a method of controlling cavitation damage in hydraulic systems while operating a hydraulic device wherein a displacing force is transmitted to a displaceable member by means of a hydraulic fluid, the improvement which comprises employing as said fluid a composition of claim 2.

i i t 

2. A composition as claimed in claim 1 wherein the viscosity index improver is a mixture of polyalkylmethacrylates, and the alkyl ester groups of (a) and (b) are different.
 3. A composition as claimed in claim 1 wherein the viscosity index improver is a mixture of polyalkylacrylates, and the alkyl ester groups of (a) and (b) are different.
 4. A composition as claimed in claim 2 wherein the phosphate ester is dibutylphenyl phosPhate.
 5. A composition as claimed in claim 2 wherein the phosphate ester is tributyl phosphate.
 6. A composition as claimed in claim 2 wherein the phosphate ester is a mixture of tributyl phosphate and tricresyl phosphate.
 7. A composition as claimed in claim 2 wherein the phosphate ester is a mixture of a trialkyl phosphate and cresyldiphenyl phosphate.
 8. A composition as claimed in claim 1 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).
 9. A composition as claimed in claim 2 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).
 10. A composition as claimed in claim 4 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).
 11. A composition as claimed in claim 5 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).
 12. A composition as claimed in claim 6 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).
 13. A composition as claimed in claim 7 wherein the viscosity index improver (a) is a poly(butylmethacrylate) and (b) is a poly(hexylmethacrylate).
 14. In the method of controlling cavitation damage in hydraulic systems while operating a hydraulic device wherein a displacing force is transmitted to a displaceable member by means of a hydraulic fluid, the improvement which comprises employing as said fluid a composition of claim
 1. 15. In a method of controlling cavitation damage in hydraulic systems while operating a hydraulic device wherein a displacing force is transmitted to a displaceable member by means of a hydraulic fluid, the improvement which comprises employing as said fluid a composition of claim
 2. 